This invention relates to nonmetallic curable compositions including a curable resin and a hardener, of particular use in composite matrix and adhesive applications.
Curable nonmetallic resin systems are widely used to form strong bonds between various types of articles. In a conventional adhesive application, curable epoxy-based resins are used to permanently bond pieces of structure together. For example, many aircraft and spacecraft structures are bonded together with curable resins rather than fasteners such as bolts or rivets, to reduce the weight of the structure and improve the performance of the joint. In another application, nonmetallic composite materials are made with a curable resin matrix bonding together reinforcement fibers. The resulting composite materials have very high modulus-to-weight and strength-to-weight ratios.
Curable resin systems are prepared by mixing together at least two components, a resin and a hardener, and then curing the mixture. The resin is ordinarily a flowable organic material which is reacted with the hardener to produce a solid. The mixture is hardened to its full strength by a process termed curing, which involves holding the mixture at some temperature to permit the polymerization reaction to proceed to completion to produce the final properties of the cured resin system. Where the cured is used as an adhesive, it is held between the articles to be bonded during curing. Where the cured resin is to be the matrix of a composite material, the reinforcement is mixed with the resin system before curing, and curing is completed.
A curable resin system must meet other requirements to be acceptable for a particular use, in addition to having the necessary mechanical properties. The resin system must wet and adhere to the articles being bonded, to ensure that the bonded structure achieves its full strength.
The application of the resin system after mixing but before curing is important. The resin system must have the proper consistency for application. It must also have a sufficiently long "pot life" in large batches so that economic amounts of the resin system can be mixed as a batch and retained without hardening prior to application. The resin system should not be too strongly exothermic upon curing, because the heat generation can be so high as to cause degradation and prove hazardous.
A critical consideration for resin systems is that they should not contain noxious or dangerous chemicals that might harm workers during the mixing and application stages. This requirement is an important consideration, as many prior systems use ingredients that are harmful when breathed or touched by humans. These prior systems may achieve operable results, but are increasingly banned from use by safety considerations and governmental regulations.
The resin system should be curable at low temperatures, to achieve low cost, good accuracy of part tolerances and therefore reproducibility, and relatively low tooling costs. When a part must be heated to elevated temperature for curing, it assumes a zero stress state at that temperature. Then, when it is cooled to lower temperature after processing and in service, the differential shrinkage between the components (such as the matrix and the reinforcement of a composite material) introduces internal stresses that can distort and/or crack the part. On a related point, generally resin materials shrink in volume when cured, and this shrinkage can introduce internal stresses in much the same manner as do differential thermal stresses. It is desirable that the resin system undergoes little if any volume change upon curing, or expand slightly.
Existing resin systems that can be cured at low temperatures, while meeting the mechanical property requirements, have shortcomings in respect to one or more of the precuring characteristics, health and safety concerns, and curing characteristics. There is a need for an improved low-temperature curing resin system that satisfies all of these requirements. The present invention fulfills this need, and further provides related advantages.